This invention relates to a biodegradable polymeric composition and more particularly to a polymeric composition which comprises a poly-R(-)(3-hydroxybutyric acid) or its copolymer produced by microorganisms and which is completely biodegradable and still has sufficient mechanical strength for practical uses.
A poly-R(-)(3-hydroxybutyric acid) (hereinafter abbreviated as PHB) is a biodegradable and biocompatible homopolyester and has prospects for various applications in which biodegradability or biocompatibility is required.
For instance, environmental pollution by waste plastics is becoming more serious and hence there is a great interest in biodegradable plastics. However, the biodegradable plastics proposed so far are either incapable of easy and inexpensive production or insufficient with respect to their mechanical, chemical, or physical properties.
PHB, too, is inadequate with respect to its physical properties in that it is stiff and brittle, and this problem has obstructed PHB from practical applications.
It has been proposed that the stiff and brittle nature of PHB can be alleviated by drawing an article of PHB such as a sheet, film, tape, or fiber after the article has been pretreated by rolling under pressure or by heating in a particular temperature range followed by cooling and maintaining the article at the cooled temperature for a short period [Japanese Patent Application Kokai (Laid-Open) No. 61-69431(1986)].
However, the improvement in the stiff and brittle nature of PHB by drawing is temporary and the resulting drawn article tends to recrystallize and become brittle with time. This tendency is particularly prominent at a relatively high temperature, e.g., in the range of 30.degree.-80.degree. C.
Another attempt to improve the nature of PHB relies on addition of a plasticizer or blending with a different polymer which serves as a plasticizer to form a polymer blend. For example, a polymer blend of PHB with a polyethylene oxide is described in Polymer, 29, 1731 (1988) and that with a polyvinyl acetate is described in Polymer, 30, 1475 (1989).
The alleviation of the stiff and brittle nature of PHB by addition of a plasticizer including a plasticizing polymer which has been proposed in the prior art is also unsatisfactory since PHB does not have a sufficient compatibility with the plasticizer, causing exudation of the plasticizer. As a result, the PHB article becomes brittle again, and if the article is left in the field, the pollution of water or soil by the exudated plasticizer may occur.
For example, when a polyethylene oxide, which is a water-soluble polymer, is used as a plasticizer, the exudated plasticizer will readily be dissolved in underground water. Therefore, the usage of this polymer is strictly regulated.
A blend of PHB with a polyvinyl acetate is still stiff and brittle at room temperature since the glass transition temperature of a polyvinyl acetate is about 38.degree. C. which is higher than ordinary room temperatures. Therefore, the desired improvement in the nature of PHB cannot be satisfactorily attained by the blend.